Traditionally, digital color images have been converted to a hardcopy output from a color printer using either an ordered dither process or an error diffusion process. Both of these processes are generally well-known in the color image reproduction arts and have been described in many prior art references including, for example, U.S. Pat. No. 4,733,230 issued to Kurihara et al, U.S. Pat. No. 4,651,228 issued to Koch and U.S. Pat. No. 4,339,774 issued to Temple, all incorporated herein by reference. More recently, error diffusion techniques have been used in combination with gray scale assignment methods to achieve high resolution and high print quality color printed images in the field of color ink jet printing. One such process is described, for example, in my now allowed co-pending application Ser. No. 278,881 filed Dec. 2, 1988 and this process is entitled "Method and System for Enhancing the Quality of Both Color and Black and White Images Produced by Color Ink Jet Printers". This application is assigned to the present assignee and is also incorporated herein by reference.
In the ordered dither method of image reproduction, the decision to print or not to print a pixel at a given pixel location (x, y) within an intensity I(x, y) depends upon the value of the dithered matrix at a given location, D(x, y), where D(x, y ) is typically an "n" by "m" matrix. These matrix valves are preassigned in a given pixel sequence along both the x and y directions of a super pixel or reference matrix or "tile", and this super pixel or reference matrix consists of a chosen number of individual pixels. If the value of I(x, y) is greater than D(x, y) at any given individual pixel location on the super pixel reference matrix, then the pixel at location x, y is printed, otherwise it is not printed.
The main advantage of this conventional approach to ordered dithering is its simplicity, since it involves only repetitive threshold comparison on a pixel by pixel basis. Different methods of selecting the dither matrix D will lead to different output characteristics, and the selection of matrix size (n by m) will also influence the output print quality. This ordered dither method of image reproduction exhibits excellent computational speed and is capable of either a good gray scale resolution or a good spatial resolution, but not both. A large ordered dither matrix D will result in good gray scale reproduction but poor spatial resolution, whereas a small ordered dither matrix D will result in good spatial resolution and a poor gray scale reproduction.
Extending these conventional ordered dither approaches to color image processing, there are as many ordered dither matrix masks as there are numbers of color planes. Regardless of how many different masks are used, there will always be a situation where a given pixel must be turned on for more than one color plane. While this situation is acceptable for some printing processes, it may not always be acceptable for other printing processes such as ink jet printing where different colors of ink, when piled on top of one another, will result in poor color mixing or undesirable color bleed. In electrophotographic processes which do not use transparent toners, different colors of toners piled on top of one another will also result in poor picture quality. In order to resolve the above conflicts with both ink jet and electrophotographic printing, a large number of computational processes are required.
Ordered dither processes of the above type are further described in a textbook entitled Fundamentals of Interactive Computer Graphics by Foley and Van Dam, Addision-Wesley Publishing Company, Copyright 1982, incorporated herein by reference. See particularly Section 17.2.2 and pages 597-602 of this reference.
Error diffusion is a technique used to distribute the remainder information to the neighboring pixels surrounding a primary printed pixel. This remainder information represents the error between a printable gray scale number and the complete input image gray scale data truly representative of a scanned image. This error diffusion process is frequently carried out using a selected one of several well-known algorithms which are described in my above identified copending application Ser. No. 278,881 and also in an article by Floyd and Steinberg entitled "An Adaptive Algorithm For Spatial Gray Scale", Proceedings of the Society of Information Display, Vol. 17/2, 1976, incorporated herein by reference.
Color and monochromatic printing methods which employ only ordered dither processing or only error diffusion processing alone have been characterized by several distinct disadvantages. Whereas ordered dither processes are very fast in computational speed, a significant amount of gray scale information can be lost due to the above described thresholding technique. These ordered dither processes have also been characterized by excessive noise, poor spatial resolution, and poor low and high frequency responses. In addition, when using multiple color planes and when restricted to dot-next-to-dot (DND) formatting, the traditional method of ordered dither is difficult to implement and is inefficient in resolving the DND formatting requirement.
On the other hand, printing methods which use only error diffusion to the exclusion of ordered dither techniques are relatively slow in computational speed (e.g. 1/8 of the ordered dither computational speed) and will render an image with an objectionable artifact known in the color printing art as "worms". This artifact is caused by allowing the error number generated in the error diffusion process to accumulate during printing and by the failure to adequately distribute the error remainder drop counts to pixels surrounding the just printed pixel or super pixel.